Direct sequence code division multiple access (DS-CDMA) is one of the effective wireless access technologies that supports high capacity, variable and high data rate transmission services in wireless communication systems. DS-CDMA has been adopted in third generation wireless communications systems.
Existing DS-CDMA systems are single carrier transmission systems. Two kinds of receivers are generally applied for such DS-CDMA systems namely, Rake receivers and time-domain equalization (TDE) receivers. The performance of these receivers depends on the properties of the wireless environment in which such receivers operate. Because of multipath delay spread of a wireless channel, inter-finger interference (IFI) and multiple access interference (MAI) are inherent with Rake receivers. On the other hand, TDE receivers, though theoretically capable of suppressing IFI and MAI, suffer from slow convergence and complicated computations when applied in DS-CDMA systems. Furthermore, TDE receivers are not able to suppress IFI and MAI effectively in practice.
Multicarrier transmission schemes have been proposed as an effective way to improve channel capacity utilization under multipath interference and frequency selective fading reception caused by multipath delay suppression. Orthogonal frequency division multiplexing (OFDM) is an effective multicarrier modulation scheme to combat the frequency selectivity of a channel using a simple one-tap equalizer. OFDM prevents inter-symbol interference (ISI) and inter-carrier interference (ICI) by inserting a cyclic prefix (CP) between adjacent OFDM symbols. Moreover, the signal can be transmitted and received using fast Fourier transform WEFT) devices without increasing transmitter or receiver complexities.
Uncoded OFDM transmission technique applied in a multipath environment has a bit error rate (BER) comparable to that of a narrow band radio channel because the fading of each sub-carrier is frequency non-selective. To overcome this behavior and to reduce the BER, a combination of OFDM and CDMA, called multicarrier CDMA (MC-CDMA), has been proposed recently. In MC-CDMA system, the energy of each information symbol is spread over several sub-carriers, which leads to a diversity gain in a broadband-fading channel. However, similar to any multicarrier modulation scheme, MC-CDMA suffers from two major implementation difficulties. First, high peak-to-average power ratio (PAPR) problem is inherent with MC-CDMA. Hence, highly linear and inefficient amplifiers must be used to avoid distortion and spectral spreading. Second, MC-CDMA is sensitive to the frequency offset and RF phase noise. These two issues limit the applicability of MC-CDMA in practical wireless environment.
In downlink communication of CDMA systems, all mobile stations within the same cell or sector make use of the same frequency band and same time slot but different spreading codes for data transmission. Spreading codes consist of two layers of codes, namely long scrambling codes and short channelization codes. The long scrambling codes are common for all mobile stations within the same cell or sector. However, the mobile stations are allocated unique short channelization codes that are orthogonal to each other. To support multi-data rate transmissions, two spreading methods may be used, namely multi-code (MC) and orthogonal variable spreading factor (OVSF) methods.
Communication channels allocated for transmissions to dedicated mobile stations are referred to as downlink traffic channels (DTCHs). In order to establish and maintain the connections between the base stations and mobile stations, a common pilot channel (CPICH) and a common control channel (CCPCH) are also allocated to each cell or sector for conveying relevant information shared by all mobile stations within the same cell or sector. Data symbols to be conveyed by DTCHs, CPICH and CCPCH are orthogonally spread, synchronously multiplexed, and transmitted through the same wireless channel. In multi-data rate transmissions, the orthogonality of the spreading codes for low- and high-data rate channels, and the spreading codes for CPICH and CCPCH are maintained.
Where there are many obstacles such as buildings and hills between a base station and a mobile station, a wireless channel is well modeled as a wide-sense stationary uncorrelated scattering (WSSUS) channel. In a CDMA downlink transmission, a transmitted signal arrives at a mobile station as several time-delayed, amplitude-scaled versions of the transmitted signal along multiple paths. The CDMA receiver of the mobile station resolves the multipath components of the transmitted signal into several paths known as rays. The rays have time delays that are multiples of the spreading chip interval Tc, and a resolved ray with time delay τ represents a group of multipath rays with time delays over the interval [τ−Tc/2,τ+Tc/2]. If there is only one resolved ray, a frequency non-selective fading channel is observed. However, if there is more than one resolved ray, a wireless channel is called a frequency selective fading channel.
For a frequency non-selective fading channel, informational data may be recovered at the mobile station using a simple despreader without any intracell interference. Practically, however, a wireless channel is a frequency selective fading channel because of large time dispersion of the multipath components. Conventionally, a CDMA receiver in the mobile station employs a Rake combiner to coherently combine despread outputs from all resolved rays of a transmitted signal that are determined by a path searcher, thereby recovering the transmitted signal.
In order to improve the performance of CDMA downlink transmissions, receivers that suppress IFI and MAI are needed. When a delay spread is large, the frequency selective fading channel may be transformed into a frequency non-selective fading channel through channel equalization. Therefore, a channel equalized receiver seems to be an effective CDMA receiver to recover transmitted data by restoring the orthogonality of spreading codes used, thus suppressing both IFI and MAI.
A CDMA system with a conventional TDE receiver requires a large number of filter taps because of the high chip rate of such a CDMA system. The large number of filter taps causes noise to increase and a convergence problem when implemented with adaptive algorithms. A least mean square (LMS) algorithm is not applicable to acquire equalizer coefficients because of the typically long convergence time of this algorithm. Algorithms that converge faster, such as recursive least square (RLS) may be too complex for applications with large number of equalization taps. If an adaptive algorithm diverges, or converges slowly, it will be difficult to achieve the suppression of IFI and MAI desired in an equalization receiver.
In order to suppress the multipath delay spread, multicarrier modulation is applied to increase the symbol duration by splitting a high-rate serial stream into multiple parallel low-rate streams. Each of the multiple parallel low-rate streams is modulated using a different sub-carrier. Orthogonal frequency division multiplexing (OFDM) is an effective multi-carrier modulation scheme because of its efficient digital implementation through FFT and inverse FFT (IFFT) and the orthogonality of the sub-carriers, even though such sub-carriers overlap. By inserting a cyclic prefix (CP) for each OFDM block, the inter-symbol interference (ISI) can be suppressed as long as the CP length is greater than the maximum excess delay of the channel.
A conventional TDE receiver for CDMA downlink transmissions typically includes a cell searcher, a code generator, a path searcher, a despreader, an equalizer, and a signal detector. The cell searcher receives, from a CDMA downlink system, a transmitted signal and the corresponding rays, and retrieves long scrambling codes and information relating to a cell from the transmitted signal. The code generator uses the long scrambling codes, generates a combination of long scrambling codes and short channelization codes, known as spreading codes, for CPICH, CCPCH and the DTCHs required by the equalizer. The path searcher then uses data symbols of the CPICH, the long scrambling codes, the short channelization codes, and the received chip signals, to provide time delay information of several rays with the largest received powers.
Generally, a chip signal from a physical channel j passes into a corresponding FIR filter of the equalizer to thereby produce a filtering output. A signal combiner then sums outputs from each FIR filter generating a signal z(n) for further processing by the despreader. Coefficients of the equalizer can be obtained, for example, by minimizing the difference, in minimum mean-square-error (MMSE) sense, between the overall equalization output z(n) and delayed version of the transmitted signal, x(n−u), where u is called the reference timing. The path searcher provides reference timing required by the equalizer during operation.
The despreader then despreads the output of the equalizer using the spreading codes from the code generator and allocated to an intended mobile station. The signal detector then recovers informational data for the intended mobile station from the output of the despreader.
Conventional equalization receiver chooses the FIR filter length G being greater than or equal to the subchannel length L, so that the energy from all taps of the channels can be captured. Only one long equalizer is required if G is chosen by this way, and the minimum number of total equalizer coefficients is ML, which can be very large for wireless mobile environment.
An equalization receiver with, large number of filter taps involves noise enhancement and convergence problem when implemented with adaptive algorithms. In fact, LMS (“least mean square”) algorithm is not applicable to acquire the equalizer coefficients, because of the typically long convergence time of this algorithm. Algorithms that converge faster, such as RLS (“recursive least square”), may be too complex for applications with large number of equalization taps. If the adaptive algorithm diverges, or converges slowly, it will be difficult to achieve the suppression of IFI and MAI, which is the very beginning object of an equalization receiver.
A multicarrier modulation scheme is robust to frequency-selective fading. However, the multicarrier modulation scheme has severe disadvantages such as difficulty in sub-carrier synchronization and sensitivity to frequency offset and nonlinear amplification. This is because multicarrier modulation uses a lot of subcarriers with overlapping power spectra and exhibits a non-constant nature in its envelope. By combining multicarrier modulation with CDMA, the symbol rate in each sub-carrier can be lowered so that longer symbol duration makes it easy to quasi-synchronize the transmissions. In other words, MC-CDMA spreads the original signal using a given spread code in the frequency domain. For MC-CDMA, it is essential to have frequency non-selective fading over each sub-carrier. Therefore, if the original symbol rate is high enough to become subject to frequency selective fading, the signal needs to be converted from serial to parallel before spreading over the frequency domain.
A transmitter for an MC-CDMA system is similar to that used in normal multi-carrier modulation except that, in normal multi-carrier modulation, the same symbol is transmitted in parallel through different sub-carriers. The input information sequence, r(m), is first converted into M parallel data sequences, s(n,0), s(n,1) . . . s(n,M−1), where n is the symbol number. Then each serial-to-parallel converter output is multiplied with the spreading code of length G, d(n,0), d(n,1) . . . D(n,N−1). All the data corresponding to the total number of sub-carriers (N=M×PG) are modulated in baseband by inverse discrete Fourier transform (IDFT) and converted back into serial data.
A cyclic prefix of length p is inserted between symbols to alleviate inter-symbol interference caused by multipath fading. The input information sequence, now a modulated prefixed baseband spread signal, is finally transmitted after radio frequency upconversion. After down-conversion and perfect synchronization, the cyclic prefix is removed. The data is converted from serial to parallel and N sub-carrier components, corresponding to the received data (y(n, k), where k=1, 2, . . . N), is coherently detected with DFT and, thereafter, equalized with the help of pilot assisted channel estimation. The equalization process combines the energy of the received signal scattered in frequency domain. The transmitted information sequence is recovered with the help of a despreading module using the spread code.
For conventional single carrier DS-CDMA systems, both Rake and TDE receivers are associated with inter-finger interference (IFI) and multiple access interference (MAI), which limit the capacity of the system.
MC-CDMA suffers from two major implementation difficulties. First, high peak-to-average power ratio (PAPR) problem is inherent with MC-CDMA, hence, highly linear (and inefficient) amplifiers must be used to avoid distortion and spectral spreading. Second, MC-CDMA is very sensitive to the frequency offset and RF phase noise. These two issues limit the applicability of MC-CDMA in practical wireless environment.
Therefore, what is clearly needed is a CDMA system that is based on single carrier modulation with improvements to both MAI and IFI suppression due to multipath delay spread.